Diesel engines are widely used in a number of applications including road vehicles and off the road vehicles and are notable for their durability and efficiency. Environmental concerns have, however, led to regulation by various government agencies. For example, the United States Environmental Protection Administration (EPA) has established a minimum cetane number requirement of 40 and a maximum sulfur content of 500 ppmw for road diesel fuels and the California Air Resources Board (CARB) has set a maximum aromatics content for commercial road diesel fuels of 10 vol. % (9.5 wt. %). As an alternative to meeting the 10% aromatic specification, CARB permits some diesel fuels with aromatics content above 10 vol. % to be produced and sold in California if it can be established that the higher aromatic diesel fuels have combustion emissions which are no worse than those of a reference fuel containing 10 vol. % maximum aromatics. Subsection g of Section 2282, Title 13, California Code of Regulations, describes the procedure for certifying diesel fuels of equivalent emissions reductions. The emissions performance of a ASTM D975 No. 2-D low sulfur diesel fuel is subjected to comparative emissions testing by the procedure set out in the regulations in a diesel engine such as a Detroit Diesel Corporation Series-60 engine (or other specific engine designated by CARB) against a low aromatics (10 vol. % max.) diesel fuel which conforms to the following product specification:
TABLE 1 ______________________________________ CARB 10% Aromatics Reference Fuel Specifications Aromatics ASTM D 1319 10 vol % Max ASTM D 5186 95 wt % max Flash ASTM D 93 54.degree. C. Min. Gravity ASTM D 287 33-39 API Natural Cetane Number ASTM D 613 48 Min. Nitrogen ASTM D 4629 10 ppmw Max. Polycyclic Aromatics ASTM D 2425 1.4% wt. % Max. Sulfur ASTM D 2622 500 ppmw Max. Distillation C., ASTM D 86 Initial Boiling Point 171-216 (340-420.degree. F.) 10% Recovered 204-254 (400-490.degree. F.) 50% Recovered 243-293 (470-560.degree. F.) 90% Recovered 288-321 (550-610.degree. F.) End Point 304-349 (580-660.degree. F.) ______________________________________
If the fuel provides equivalent emission benefits to the reference fuel, i.e. is at least as good as the reference fuel in emissions performance, it can be certified by CARB for sale in California: fuels equivalent to the certified fuel, i.e., diesel fuels having at least the cetane number of the certified candidate fuel, with a sulfur content, aromatic content, polycyclic aromatic hydrocarbon content, and nitrogen content no greater than the "certified" candidate fuel, can be legally sold. Because both EPA and CARB regulations limit sulfur content the sulfur content of the fuel has to be greater than 500 ppmw, limiting it to the low sulfur 2-D diesel fuel specification.
Diesel fuels which are stated to be in compliance with the CARB regulations are described in U.S. Pat. Nos. 5,389,111 (Nikanjan/Chevron), 5,389,112 (Nikanjan/Chevron) and 5,792,339 (Russell/Tosco).
Diesel fuels with good performance properties, especially of particulate emissions, and which are suitable for underground mining operations are described in EP 687 289.
While the diesel fuels described in the patents identified above represent approaches to the problem of providing diesel fuels for road and mining vehicles with improved emission characteristics, other problems remain. Cetane number is a generally accepted indicator of diesel fuel ignition quality, with higher cetane numbers representing improved ignition quality. Cetane number improvement can generally be achieved in one of two ways. Cetane number improving additives are known which can be added to various basestocks to improve the ignition quality. These additives in general, however, are nitrates, for example, octyl (normally 2-ethylhexyl) nitrate which increase the nitrogen content of the fuel and lead to increased emissions of nitrogen oxide (NOx). The alternative to using cetane number improvers is to use a fuel stock of higher intrinsic cetane number and stocks of this kind can generally be characterized as paraffinic stocks, with the straight-chain paraffins having the highest cetane numbers. See Kirk Othmer, Encyclopedia of Chemical Technology, 4th Ed. page 290, also Modern Petroleum Technology, 4th Edition, G. D. Hobson (Ed), Applied Science Publishers Limited, Barking U.K., ISBN 0853344876 (page 618). The highest cetane numbers in diesel fuel stocks are therefore obtained with blends which have a high content of normal paraffins but the problem with these stocks is that they will have poor low temperature flow properties. For example, the pour point and cloud point of these products will be relatively high as a result of the presence of the straight-chain paraffins. High cetane diesel fuels are therefore unlikely to be useful in cold climates as long as they contain high levels of the waxy straight-chain paraffins.